Benzalkonium chlorides (BACs) are quaternary ammonium compounds (QACs) which are extensively used in domestic and industrial applications and end up in natural systems. As they are bactericidal, they possess a high risk to living organisms in the environment. The 5- and 15-minute acute
toxicity of three benzalkonium chlorides (BAC) - dodecyl (C12BDMA-Cl), tetradecyl (C14BDMA-Cl) and hexadecyl (C16BDMA-Cl) benzyl dimethyl ammonium chloride - to the bioluminescent marine bacterium Vibrio fischeri was investigated using the standard Microtox®
assay. The 5- and 15-minute EC50 value for all BACs tested in this study was below 1 mg/L. The lowest EC50 value indicating the highest toxicity was found for C12BDMA-Cl, followed by C14BDMA-Cl and C16BDMA-Cl. This result indicates that
BAC toxicity decreases with increasing alkyl chain length or hydrophobicity and is well-correlated with both the critical micelle concentration (CMC) and the 1-octanol/water partition coefficient (Kow) determined in this study. The effect of counter ions, such as bromide, nitrate
and acetate, on the toxicity of BACs was investigated at 342 mM counter-ion concentration. None of the counter-ions tested affected significantly the toxicity of the BACs at both exposure times (p > 0.05). This result indicates that BACs are toxic at such low concentrations that counter-ions
do not affect their (bio)availability. The effect of natural organic matter (NOM) on the toxicity of BACs was tested at NOM concentrations up to 100 mg/L. NOM decreased the toxicity of BACs for both the 5- and 15-minute exposure time. The most pronounced effect was found for the C16BDMA-Cl,
which is the most hydrophobic QAC among the three BACs tested. The results presented here contribute to a better understanding of the environmental risk associated with benzalkonium chlorides by taking into account environmental factors, such as counter ions and NOM, as well as their interactions
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